This invention relates to cylinders for internal combustion engines. More particularly, this invention relates to methods and apparatus for reducing the distortion of cylinders during their manufacture and use.
The problems and disadvantages of prior art cylinders are best understood by reference to FIGS. 1 through 2a. FIGS. 1, 2 and 2a depict a typical prior art housing for an air cooled single cylinder internal combustion engine. Referring to FIGS. 1, 2 and 2a, housing 10 includes a cylinder 12 defined by a cylinder wall 14. A cylinder bore 16 is disposed within cylinder 12 and is adapted to receive a reciprocating piston (not shown). The piston reciprocates along a longitudinal axis 24 of cylinder bore 16. A cylinder head (not shown) is bolted onto cylinder 12 by a plurality of bolts that are received in bolt bosses 18.
Projecting outwardly from cylinder 12 is a plurality of spaced fins or air vanes 20 which assist in heat dissipation from the air-cooled engine. Housing 10 also includes a crankshaft axis 22 about which a rotatable crankshaft rotates. The piston is connected to the crankshaft by a connecting rod (not shown).
As depicted in FIGS. 1 through 2a, the thickness of cylinder wall 14 is substantially uniform both in the direction parallel to longitudinal axis 24 of cylinder bore 16 (FIG. 1) and in the direction along the planes substantially normal to longitudinal axis 24 (FIGS. 2 and 2a).
Except in those instances when the thickness of wall 14 is very large, it has been found that the use of a typical uniform wall thickness results in the distortion of cylinder bore 16 from its ideal cylindrical shape. There are at least two primary causes for this bore distortion. First, the torquing of the cylinder head bolts into bolt bosses 18 during the manufacturing process introduces stresses to the engine housing material, resulting in distortion of the cylinder bore. The cylinder bore becomes elliptical, with the major axis of the ellipse being perpendicular to the piston's longitudinal axis and parallel to the crankshaft axis.
The second major cause of bore distortion is due to the thermal expansion of the bore during engine operation. This distortion induces expansion stresses into the engine housing material which, when coupled with the above-described stresses, causes yielding in the housing material and forces a permanent deformation (out of roundness) of the cylinder bore.
One major disadvantage of the cylinder bore distortion is that exhaust emissions are increased. Since the piston and rings have a substantially circular shape in cross section and the bore has a non-circular shape in cross section during engine operation due to bore distortion, it is apparent that small gaps will exist between the piston and ring outer wall surface and the bore inner surface. These gaps permit oil and gasses from the crankcase to leak into the combustion chamber and eventually out the exhaust valve into the environment. Exhaust emissions are thus undesirably increased due to the cylinder bore distortion.
One prior art way to substantially eliminate the cylinder bore distortion has been to uniformly and substantially increase the thickness of cylinder wall 14 along the entire length and the entire circumference of the cylinder. Although this method is effective, it greatly increases the amount of material that must be used in forming the engine housing. Particularly when the engine housings are mass produced in many thousands or even millions of units, the additional cost of providing such cylinder wall reinforcement is quite substantial.
Another cause of cylinder bore distortion is believed to be related to the shape of air vanes 20. As best shown in FIGS. 2 and 2a, air vanes 20 are typically asymmetrical in shape having varying radii of curvatures. In FIG. 2a, air vane 20 has a much greater length at section 21 than at section 23. Also, air vane 20 abruptly terminates at corners 25 and 27. As a practical matter, the air vanes are often designed to provide as much surface area for air cooling as space limitations will permit.
One disadvantage of using asymmetrical air vanes is that cylinder cooling will be different at different points along cylinder wall 14, depending upon the length and the shape of the fins nearest that point. For example, cylinder cooling will be different near section 21 than near section 23 (FIG. 2a) because air vane 20 has a much greater length at section 21 than at section 23. This unevenness in cylinder cooling is believed to be a cause of cylinder bore distortion.
Also, the degree of change in air vane length contributes to bore distortion. That is, the more abrupt the change, the greater the bore distortion. See corners 25 and 27 in FIG. 21, where the vane size changes abruptly.